In heavy utility vehicles, a diesel engine, which drives the rear axle via a cardan shaft, is generally installed above the front axle. As a result of this arrangement, the load on the rear axle depends greatly on the loading of the utility vehicle. For this reason, the brake force which is applied by the driver via the brake pedal at the rear axle is limited in conventional brake systems by a pneumatic valve with an axial load sensor (ALB, automatically load-dependent brake). This ensures that in the case of an unladen vehicle the entire brake pressure which is desired by the driver is not present at the rear axle, thereby reducing the tendency of the rear axle to lock. Furthermore, a relatively high level of braking comfort is made possible and the service life of the rear axle brake is extended.
In addition to the automatically load-dependent brake (ALB), contemporary utility vehicles have an anti-lock brake system (ABS). This has a rotational speed sensor (inductive sensor) on each wheel, and a pressure control valve which can modulate the pressure set by the driver by the brake pedal. The ABS is activated when there is a tendency of a wheel to lock.
What is referred to as an EBD (electronic brake force distribution) function is discussed in the prior art, for example, DE 10 2006 045 317 A1, said EBD function implementing the automatically load-dependent brake (ALB function) using ABS pressure control valves, therefore eliminating the need for the load valve. Since, in contrast to the automatically load-dependent brake (ALB), the ABS does not have a load sensor, the loading state is determined during braking using the speed difference between the front and rear axles (vVA-vHA). The lower the rear axle load, the more slip occurs at the rear axle with the same brake activation, i.e. the greater the speed difference vvA-vHA. The difference between the laden and unladen vehicle has a significantly smaller effect at the front axle because of the arrangement of the engine. The speed difference vVA-vHA during braking can therefore be used as a measure for the loading. In other words, during a braking process, EBV shuts off the rear axle brakes, and under certain circumstances also an individual rear wheel, from the further build-up of pressure, depending on the differential slip between the front and rear axles through activating the ABS valves.
However, if the front axle brakes are defective, that is to say the front axle brakes more weakly than normal or even not at all, the speed difference vVA-vHA can become very large. The EBD function would then set the ABS valves at the rear axle to “maintaining pressure” and limit the brake pressure at the rear axle. However, in this case the utility vehicle would be underbraked. Furthermore, in the case of a vehicle which is laden only to a very small degree or unladen, the problem arises that the brake pressure control is not very sensitive since in this case the differential slip between the front and rear axles becomes relatively large and a predefined differential slip threshold value is quickly exceeded.
In contrast, the present invention is based on the object of further developing a method of the type mentioned at the beginning in such a way that a more sensitive brake pressure control with at the same time a shorter braking distance is obtained.
This object may be achieved according to the invention by the features described herein.